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Exertional Myopathies: How to Care for Horses with Muscle Issues

It’s common for a horse suffering from any kind of exertional myopathy to experience muscle pain, fatigue, cramping, damage and other muscle issues. However, despite it being fairly common, using special alterations to your horse's diet and exercise routine, this condition is manageable.

RER

Recurrent exertional rhabdomyolysis (RER) or "tying-up" is believed to result from abnormal calcium cycling in the equine body system, a process that's responsible for controlling the contraction and relaxation of muscles. The most visible sign of acute RER is a horse's reluctance or refusal to move since the disease causes muscles to be hard, stiff and painful. Standardbreds, Thoroughbreds, and Arabians tend to be predisposed to RER, with the condition often affecting fit animals, especially if they're stressed or after an exercise session.

PSSM

There are two types of polysaccharide storage myopathy (PSSM). Type 1 is caused by the mutation of the gene responsible for regulating the production of glycogen synthase, leading to an abnormal accumulation of glycogen in muscle cells. Type 2, which has no medically known cause, is often diagnosed through muscle biopsy that reveals unusual clumping of glycogen in muscle cells, yet with no gene mutation like in Type 1 PSSM.

Special Consideration on Feeding & Exercise

First thing; reduce non-structural carbohydrates (NSCs) like sugar and starch in the affected horse's diet, while simultaneously raising calorie levels by providing more fats in his diet. According to Stephanie Valberg, DVM, PhD., Dipl. ACVIM, ACVSMR of the College of Veterinary Medicine’s Department of Large Animal Clinical Sciences at Michigan State University, RER-afflicted horses shouldn't have more than 20% of their calories provided by NSCs. This helps reduce excitability and raising calorie uptake from fats to at least 20%.

Equine body condition also matters, with horses getting 5kg/11lbs or higher of grain being at higher risks of contracting RER than those consuming 2.5kg/5.5lbs or lower.

For horses with Type 1 PSSM, Valberg recommends only 10-15% of total diet to be NSCs. Since insulin stimulates the secretion of glycogen synthase, this reduction of insulin in circulation helps control levels of glycogen in muscle cells. However, going too low would also negatively impact on insulin secretion.

As for fats, the amount required depends on a specific horse’s body condition and exercise requirements. Although PSSM horses have traditionally been provided with similar amounts, recent research shows that Type 2 PSSM horses often exhibit much lower muscle glycogen build-ups, meaning they may not require strict NSC restriction like Type 1 horses. However, both groups may need an amino acid/protein supplement if they've decreased muscle mass.

Further, Valberg recommends routine daily exercise (about 15 minutes of riding and/or round pen work) for Quarter Horses suffering from PSSM. As for Warmbloods and English horses with Type 2 PSSM, riding exercises involving long and low warmups help relax neck muscles, lift their backs, as well as engage abdominal and core muscles to minimize muscle soreness and reduce reluctance to move forward and work under saddle.

QUICK TIPS:

- These changes to exercise routines need to be introduced gradually. Further, walk breaks need to be interspersed with work intervals to build up proper strength.